1. Field of Invention
The invention relates to an amplifier and, in particular, to a high frequency amplifier with trans-impedance amplifiers and a DC Feed-Forward circuit structure used in a high-speed optical communication interface.
2. Related Art
The trans-impedance amplifier (TIA) is a device with a single terminal output. When the single terminal output goes through an amplifier and changes to differential terminal outputs, an additional DC voltage is required. The DC voltage and the output DC voltage of the TIA will affect the performance of the amplifier. Both of the DC voltages have to be appropriately controlled in order to maintain the amplifier's normal performance.
The TIA in the prior art shown in FIG. 1 consists of a first amplifier 11, a feedback resistor 12, a low-pass filter 13, and a second amplifier 14. The voltage output from the first amplifier 11 enters the first terminal A of the second amplifier 14. The low-pass filter 13 extracts the DC voltage of the first terminal A and sends it to the next gain stage. Although the circuit structure in FIG. 1 can provide a balanced DC voltage to the next differential gain stage, the second terminal B is affected by the low-pass filter 40. Both the first terminal A and the second terminal B have different reactions to the power supply source, which in turn affects the response of the TIA to PSRR.
Another structure of the TIA in the prior art is illustrated in FIG. 2. It consists of a first amplifier 21, a second amplifier 22, a third amplifier 23, a first feedback resistor 24, a second feedback resistor 25, and a variant current source 26. This structure fixes the reference voltage on the second terminal B of the third amplifier 23. The DC voltage on the first terminals A changes with the input current Iin. Therefore, changing the strength of the variant current source 26 modifies the DC voltage on the first terminal A, making the DC voltages of the terminals A and B consistent.
Some technical problems existing in the structure of FIG. 2 have to be solved. For example, increasing the transmission speed makes the first and second feedback resistors 24, 25 decrease continually. The DC voltage variation on the first terminal A also decreases. If differences in the manufacturing processes are also taken into account during the design, the second terminal B has to cover the offset state. This will make it difficult to set the DC voltage. The incorrect reference voltage will make the variant current source 26 dormant or active with a very little signal. The former situation results in the fact that the first terminal A cannot follow the voltage on the second terminal B, whereas the latter situation affects the sensitivity of the TIA. The worst case will render the TIA totally ineffective.
When using the amplifiers shown in FIGS. 1 and 2 in a high-frequency circuit or a high-frequency optical communication interface, one also faces the problem of noises that is hard to overcome.